And lots of people won't eat red meat or pork but do eat poultry and/or seafood.

Less commonly practiced is the form of vegetarianism known as veganism. A vegan (pronounced: VEE-gun) doesn't consume any animal-derived foods or use animal products or byproducts, and eats only plant-based foods.

In addition to not eating meat, poultry, seafood, eggs, or dairy, vegans avoid using products made from animal sources, such as fur, leather, and wool.

While those are obvious animal products, many animal byproducts are things we might not even realize come from animals. These include:

gelatin (made using meat byproducts)

lanolin (made from wool)

rennet (an enzyme found in the stomach of calves, young goats, and lambs that's used in cheese-making)

Why Vegan?

Veganism (also known as strict vegetarianism or pure vegetarianism), as defined by the Vegan Society, is "a philosophy and way of living which seeks to exclude — as far as is possible and practical — all forms of exploitation of, and cruelty to, animals for food, clothing, or any other purpose."

Vegans also avoid toothpaste with calcium extracted from animal bones, if they are aware of it. Similarly, soap made from animal fat rather than vegetable fats is avoided. Vegans generally oppose the violence and cruelty involved in the meat, dairy, cosmetics, clothing, and other industries.

What About Nutrition?

According to the Academy of Nutrition and Dietetics (AND), "appropriately planned vegetarian diets, including total vegetarian or vegan diets, are healthful, nutritionally adequate and may provide health benefits in the prevention and treatment of certain diseases. Well-planned vegetarian diets are appropriate for individuals during all stages of the life-cycle including pregnancy, lactation, infancy, childhood and adolescence and for athletes."

Vegetarian diets offer a number of advantages, including lower levels of total fat, saturated fat, and cholesterol, and higher levels of fiber, magnesium, potassium, folate, and antioxidants. As a result, the health benefits of a vegetarian diet may include the prevention of certain diseases, including heart disease, diabetes, and some cancers.

But any restrictive diet can make it more difficult to get all the nutrients your body needs. A vegan diet eliminates food sources of vitamin B12, which is found almost exclusively in animal products, including milk, eggs, and cheese. A vegan diet also eliminates milk products, which are good sources of calcium.

Vitamin D. Vitamin D helps our bodies absorb calcium and is synthesized by exposing skin to sunlight. But vitamin D deficiency can occur, especially if you don't spend a lot of time outside. Vitamin D is not found in most commonly eaten plant foods; the best dietary sources are fortified dairy products. Vegans also can get vitamin D from fortified foods, including vitamin D-fortified soy milk, rice milk, almond milk, orange juice, and some cereals. Vitamin D2 supplements are plant-derived, whereas most vitamin D3 is derived from animal products.

Protein. Not getting enough protein is a concern when switching to a vegetarian diet. Protein needs can be met while following a vegan diet if you consume adequate calories and eat a variety of plant foods, including good plant sources of protein such as soy, other legumes, nuts, and seeds.

Iron. Iron from plant sources is less easily absorbed than iron in meat. This lower bioavailability means that iron intake for vegetarians should be higher than the RDA for nonvegetarians. Vegetarian food sources of iron include soy foods like soybeans, tempeh, and tofu; legumes like lentils and chickpeas; and fortified cereals. Iron absorption is enhanced by vitamin C.

Zinc. Zinc plays a role in many key body functions, including immune system response, so it's important to get enough of it, which vegans can do by eating nuts, legumes, miso and other soy products, pumpkin and sunflower seeds, tahini, wheat germ, and whole-grain breads and cereals.

Omega-3 fatty acids. The omega 3 fatty acids (DHA, EPA, and ALA) are important for cardiovascular health and brain function. DHA and EPA are found in fish, eggs, and algae. Vegans can get these essential fatty acids through a diet rich in alpha linolenic acid (ALA), a plant-based omega-3 fatty acid. ALA is found in flaxseed, chia seed, walnuts, canola oil, soy. DHA from microalgae can be found in supplements and fortified foods.

Eating a Vegan Diet

Anyone following a vegan diet has to be a meticulous label-reader. No federal regulation dictates the use of the words "vegetarian" or "vegan" in the United States. To be sure a food truly is "suitable for vegans," check the label — what might be vegetarian isn't necessarily vegan.

Vegans are by no means stuck eating boring foods with little variety. But if you're considering becoming a vegan, or wondering whether it's realistic to eliminate animal-based foods from your diet, it might pay to start slowly, especially if you've been a cheeseburger fan most of your life.

Try some of the wide array of meat alternatives that are found in almost every grocery store. Tasty frozen veggie burgers, chicken and meat substitutes, sausage alternative, fake bacon, and tofu dogs will make the transition to a vegan diet convenient and easy.

If you need help, talk to a registered dietician familiar with vegan diets and look for vegetarian cookbooks that can help you plan and prepare healthy meatless meals.

And remember, many foods you probably already have are suitable for a vegan diet. For instance, most breakfast cereals are vegan as are many crackers, cookies, and baked goods. Choose ones made with whole grains and low in fat, pair them with healthy salads, fresh fruits, and some colorful veggies, and you might not ever miss that ham and cheese sandwich!